1. Technical Field
The present invention relates to a semiconductor device and a method of manufacturing the same.
2. Background Art
With recent advancement in shrinkage of LSI, degradation in operation current due to depletion of polysilicon gate electrode, which composes MOSFET (Metal-Oxide Semiconductor Field Effect Transistor), has been becoming increasingly problematic. Investigations have, therefore, been made on techniques of avoiding depletion of the electrode, by using metal gate electrodes.
As a MOSFET making use of a metal gate electrode and a high-k insulating film, there has been known a structure having a gate insulating film, a metal gate electrode, and a polysilicon gate electrode stacked on a semiconductor substrate. One exemplary MOSFET making use of such structure is described in Japanese Laid-Open Patent Publication No. 2007-208260. An N-channel MOSFET described in the cited patent publication has a gate insulating film which is configured by a SiO2 film formed over a semiconductor substrate and a high-k insulating film stacked thereon, and a gate electrode which is configured by a lower electro-conductive film and an upper electro-conductive film typically composed of polysilicon stacked thereon. On the other hand, a P-channel MOSFET has a gate insulating film which is configured by a SiO2 film formed over the semiconductor substrate and a high-k insulating film stacked thereon, and a gate electrode which is configured by a lower electro-conductive film, an insulating film, an intermediate electro-conductive film, and an upper electro-conductive film typically composed of polysilicon stacked in this order. Materials composing the lower electro-conductive film are exemplified by metals such as Ti, Al and Ta, and metal nitrides such as TiN. Materials composing the insulating film are exemplified by silicon oxide film, silicon nitride film, and metal nitride films typically composed of Al, La and Y. Materials composing the intermediate electro-conductive film are exemplified by metals such as Ti, Al and Ta, and metal nitrides such as TiN. According to the description, by virtue of this configuration, the work functions of the N-channel MOSFET and the P-channel MOSFET may independently be controllable.